Can not close editor window not in GUILayout.somemethod in OnGUI method

There are things in processs o programming that you can not predict cause you did not do this kind of work. It can be editor scripting or something else. In my case, it is.

So, basically here is a moment that I would like to understand.

I have next classes: LevelBuilder - Monobehaviour class. LevelBuilderEditor - Custom editor for LevelBuilder. BuildLevelWindow - EditorWindow class.

So, it works like this: When I click on button "Build Level" in LevelBuilder inspector, I open window "BuildLevelWindow".

Then I click "Build" button. Then I start IEnumerator(Yes, it possible in edit mode, with using delegate EditorApplication.update) that generate some data.

In BuildLevelEditor In GUI I show progress bar with progress. When my Ienumerator ends, I set boolean variable called "toCloseBuildLevelWindow".

And that is it:

In OnGUI() method of class BuildLevelWindow I check when that variable will be true, and then I call Close() method and set variable to false.

BUT, not like all cases, when I call method Close() in GUILayout.Button("") blocks of code, this call throws an error in console. And that error this:

Error: GUI Window tries to begin rendering while something else has not finished rendering

Butm if I do this checking on variable in Update method of BuildLevelWindow, it works properly.

So, hopefully, someone can tell me what is this all about ?